To understand the background of the present invention, it is necessary to explain two conventional technologies in different technological fields. One belongs to a technological field of apparatuses for measuring the hardness of a tissue material of a living body using a mechanical/electrical vibration system and the other belongs to a technological field concerning CT values of a tissue material of a living body using an X-ray CT scanner. The former will be explained first and then the latter will be explained.
One conventional method for measuring the hardness of a tissue material of a living body is a method that presses a probe against the material to be measured, applies vibration thereto, detects mechanical/electrical response of the tissue material of the living body to the input vibration using a sensor and obtains characteristic values corresponding to the hardness based on variation in the frequency and phase, etc. Examples of this method are disclosed in, for example, “Measurements of the Hardness of a Soft Material with a Piezoelectric Vibrometer and Their Analysis” (Sadao Omata, Medical Electronics and Bioengineering, Vol. 28, No. 1, 1990, pp 1-8) and “New tactile sensor like the human hand and its applications” (S. Omata et al, Sensors and Actuators A, 35 (1992) pp9-15). FIG. 3 shows an apparatus for measuring the hardness of tissue of a living body disclosed in Japanese Patent Laid-Open Publication No. Hei 9-145691 as a conventional example. FIG. 3 shows a probe unit 2 pressed against the material 1 to be measured of the living body, for example, a tissue of the skin, or a tissue of viscus, such as liver tissue. The probe unit 2 includes a vibrator 3 and a strain detection sensor 4, and is connected to an external control unit 5. The control unit 5 has a self-excited oscillation circuit 6 and the self-excited oscillation circuit 6 has an amplifier 7 and a gain variation correction circuit 8. Furthermore, the self-excited oscillation circuit 6 is connected to a frequency measuring circuit 9 to measure its frequency and a voltage measuring circuit 10 to measure its amplitude.
The operation of this conventional example will be explained below. When the probe unit 2 is pressed against the material 1, the vibrator 3 inside the probe unit 2 generates self-excited oscillated vibration as an electric signal is converted to mechanical vibration by a mechanical/electrical vibration system of the vibrator 3 and the self-excited oscillation circuit 6 in the control unit 5, and the vibration is input from the end of the probe unit 2 to the material 1. The material 1 responds to this input vibration, according to its mechanical vibration transmission characteristics. The strain detection sensor 4 detects this output vibration (strain) and converts it to an electric signal. The vibrator 3 and the strain detection sensor 4 can be implemented by, for example, a piezoelectric vibration element and a piezoelectric sensor. The vibrator 3 and the strain detection sensor 4 together with the amplifier 7 form a feedback loop and then osculation is self-excited. Here, as a result of the material 1 responding to the input signal from the vibrator 3, generally the frequency changes, a phase difference is generated and the amplitude is reduced, but the gain variation correction circuit 8 has a function of correcting the amplitude gain of the output signal of the strain detection sensor 4. Furthermore, because the gain variation correction circuit 8 is formed in the feedback loop of the self-excited oscillation circuit 6, feedback is provided in such a way that the phase difference generated becomes to zero while the amplitude gain is being corrected. When the phase difference is fed back to zero, the resonance frequency of the mechanical/electrical vibration system, which includes the material 1, the vibrator 3, the self-excited oscillation circuit 6, and the strain detection sensor 4, can be obtained by the frequency measuring circuit 9.
As is well known, because this resonance state is a speed resonance state, the resonance amplitude reaches its maximum value when the phase is zero, irrespective of the damping constant of the system; this is unlike a displacement resonance state or an acceleration resonance state. Therefore, as irrespective of the damping constant, it is possible to calculate a spring constant of the system by obtaining the resonance frequency when the phase is zero. Therefore, a frequency variation df between this resonance frequency and the frequency when the probe unit 2 is not in contact with the material 1 is the characteristic value corresponding to the hardness of the material 1, that is, the spring constant of the material. For example, FIG. 4 shows a relationship between the frequency variation df and the shear modulus measured by another method in the case of a gelatin of 30 mm in thickness. FIG. 5 shows values of frequency variation df of various materials containing tissues of the living body using foam rubber as a reference. From the frequency variation df in measured in this manner, it is possible to calculate the hardness of the material 1 based on the correspondence with the shear modulus G, the correspondence with the Young's modulus using a known relational expression, and the correspondence with a spring constant of the material 1 against which a specimen having a certain diameter is pressed.
Then, a CT value of a tissue of a living body obtained using an X-ray CT scanner will be explained. The X-ray CT scanner is an apparatus that scans an object such as the tissue of the living body while irradiating the object with X-rays from an X-ray tube and relatively moving with respect to the object, measures the amount of X-rays that have passed through the object using a detector such as a Xe gas ion chamber system and obtains a position, a shape and a size of an organ inside the object using three-dimensional image processing. The basic principle of this measurement is based on the characteristic that attenuation of X-rays passing through a material varies depending on the material, that is, photons introduced into the tissue of the living body measured show a specific attenuation under the influence of the interaction with atoms of the material making up the tissue of the living body, that is, the interaction such as the photo-electric effect and Compton effect, etc. For an X-ray characteristics expression of an organ, etc., an X-ray absorption value of the material generally converts to a numerical value relative to the value of water which is considered to be zero, and this value is used as a CT value. Therefore, the CT value is represented in the following expression (1):CT value={(an attenuation factor of various materials)/(an attenuation factor of water)}−1  (1)
The unit of the CT value is expressed in Hounsfield unit (H.U.). FIG. 6 shows an example of reported CT values of various tissues of the human body. The CT value of the bone is determined by its inorganic component, and CT values of other organs are determined by components such as protein, water, and fat. That is, over an entire wide area irradiated with X-rays, the X-ray CT scanner can obtain information with high spatial resolution of the detector, not only such as the position, the size, and the shape of the tissue in the living body, but also such as X-ray absorption characteristics of the tissue of the living body from CT values, and variation of the tissue of the living body from CT value variation.
The apparatus for measuring hardness in the above-described conventional example can detect frequency variation which is characteristic values corresponding to the hardness through the function of the gain variation correction circuit as sufficiently large magnitude, and can thereby measure the hardness with fewer errors from a soft tissue to a hard tissue of the living body using a single apparatus. However, because such measurement is performed with the probe unit pressed against the material to be measured, the hardness is obtained as an average value of the area measured. Therefore, when the measurement is performed using a large probe, there is a limit of the spatial resolution such as positions. However, when the measurement is performed using a small probe, it is necessary to repeatedly change positions from one place to another to obtain data of a wide area. For example, when there a hardened area of extremely small size is present in an organ being scanned, it is difficult to specify the micro area from the wide area using a large probe, while it will take a considerable amount of time for a small probe to find the area. This problem hampers configuration of an apparatus for measuring the hardness of the tissue of the living body with high spatial resolution over a wide area.
On the other hand, the CT value obtained from the X-ray CT scanner has high resolution in three-dimensional space. However, the CT value expresses the X-ray absorption characteristic relative to water and the CT value alone cannot express flexibility and rigidity such as the hardness of a material, and it is therefore not possible to express the correspondence with the hardness of abnormalities of tissue of a living body, such as those found by palpation.
The present invention advantageously solves the above-described problems of the conventional technology and make it possible to measure the hardness of a material having high spatial resolution over a wide area, especially the hardness of tissue of a living body, such as organ tissue, and to further measure the hardness of a micro area of the entire organ. The present invention also advantageously provides an apparatus for measuring the hardness of the material capable of expressing flexibility and rigidity of the tissue of the living body from the CT value of the X-ray CT scanner. The present invention still further advantageously provides a recording medium capable of outputting the hardness data of such a material using a computer.